Thin film solar cells provide several advantages especially when used in building integrated photovoltaic applications—i.e. devices that are integrated into building structures for example as roofing shingles or exterior wall coverings. One such advantage is that these compositions have a high cross-section for absorbing incident light. That is, photovoltaic layers that are very thin can absorb and convert to electricity a relatively high percentage of incident light. For example, in many thin film solar cell devices, photovoltaic layers may have a thickness in the range of from about 1 μm to about 4 μm. These thin layers allow devices incorporating these layers to be flexible.
The conventional approach to assembly of photovoltaic cells is the so-called string & tab method, in which solar cells are connected to each other using tin or solder coated flat wire (buss) ribbons and bonded by soldering and/or other adhesive material such as conductive epoxy. The wire ribbon is typically bonded to buss bar locations on a conductive grid that is applied to the surface of the cell. It is believed that the cross section of the wire may be limited such that thicker wires are too stiff and thin and wide wires obscure too much light. The net result is that interconnect resistance losses as well as the amount of active cell surface area that is blocked by the ribbon can account for significant reduction in photovoltaic cell assembly (hence the PV device) performance. The string and tab process is particularly suited for use in photovoltaic applications, wherein the photovoltaic cells are rigid. This process may be difficult to use with thin film solar cells because the resulting series string of cells may be fragile and susceptible to lost contact of the PV ribbon with the solar cell. Furthermore, the appearance of the large buss ribbons on the surface of the PV device may be aesthetically undesirable to customers.
One approach to interconnecting thin film solar cells is the so-called “shingling” method, wherein the bottom conductive surface of one solar cell contacts the top surface of an adjacent cell. Again, this process may result in solar cell interconnects that are susceptible to lost contact between the devices.
Among the literature that can pertain to this technology include the following patent documents: U.S. Pat. No. 6,936,761; U.S. Pat. No. 7,022,910; U.S. Pat. No. 7,432,438; U.S. Publications 2007/0251570; 2009/00025788; and 2009/0255565, all incorporated herein by reference for all purposes.